1. Field of the Invention
The present invention relates to a surface emitting laser module, an optical scanner, and an image forming apparatus.
2. Description of the Related Art
A VCSEL (Vertical Cavity Surface Emitting LASER) is a semiconductor laser that emits light in a direction perpendicular to a substrate and has characteristics in that it can be manufactured at low cost compared with an end-surface emitting laser and can be easily arrayed. Therefore, it has been studied that the VCSEL is used as a light source for an optical communication such as an optical interconnection, a light source for an optical pickup, a light source of an image forming apparatus such as a laser printer, or the like. Further, the VCSEL has been proceeding toward practical utilization in some fields.
Meanwhile, an optical system, which generally has a semiconductor laser element or the like including a surface emitting laser element, has a problem in that a light amount fluctuates due to return light produced when reflection light from a lens or the window glass of a package returns to the original laser element. The fluctuation of the light amount occurs in various ways; the light amount fluctuates at a high speed of a nsec order or fluctuates at a msec order depending on the situations. Up until now, it has been assumed that the VCSEL is free from return light because it has high mirror reflectance. However, it has been confirmed as a result of a study that the VCSEL is not necessarily free from return light. Particularly, in the case of a surface emitting laser array in which plural surface emitting lasers are arranged, light emitted from one of the surface emitting lasers turns into return light and the return light is incident on an adjacent one of the surface emitting lasers, which results in the fluctuation of a light amount.
In order to address the problem due to return light, there is a method of forming a reflection preventing film on the window glass of a package. However, even if the reflection preventing film is formed to set the transmissivity of light at 99.5% or more, reflectance cannot be completely reduced to zero. Therefore, the problem due to return light cannot be totally solved.
Further, Patent Document 1 discloses another method for addressing the problem. According to this method, as shown in FIG. 1, a window glass arranged on the side of the light emitting surface of a laser element is inclined with respect to the light emitting surface. Specifically, a sub-mount 902 is provided on a metal stem 901, and a surface emitting laser chip 903 is provided on the sub-mount 902 together with a monitoring photo detector 904. The surface emitting laser chip 903 and the monitoring photo detector 904 are electrically connected to pins 905a and 905b, respectively, by wire bonding or the like. A metal cap 907 having a transparent window glass 906 through which light is emitted from the surface emitting laser chip 903 is mounted on the metal stem 901, and the metal cap 907 is bonded to the metal stem 901 by welding. Note that the window glass 906 is attached to the metal cap 907 in a state of being inclined with respect to the front surface of the surface emitting laser chip 903. Accordingly, although the light emitted from the surface emitting laser of the surface emitting laser chip 903 is reflected by the window glass 906, it never returns to the surface emitting laser because the window glass 906 is inclined with respect to the front surface of the surface emitting laser chip 903. As a result, the problem due to return light is not caused.
However, the method disclosed in Patent Document 1 is not appropriate because a number of electrodes are required and a metal stem and a metal cap are upsized in a case where a surface emitting laser array in which plural surface emitting lasers are formed is mounted. Moreover, since a surface emitting laser array requires a large area to emit light, a large glass substrate or the like must be used as a window glass. Therefore, the method disclosed in Patent Document 1 is not adapted to the surface emitting laser array. Furthermore, in terms of reliability, a surface emitting laser module in which a surface emitting laser array is packaged in an air-tight state has been demanded.
Patent Document 1: JP-A-2007-103576